1. Field of the Invention
The present invention relates generally to semiconductor processing, and more particularly to preventing lateral diffusion of silicon during the formation of a titanium silicide layer.
2. Description of the Prior Art
An important technique for reducing the scale of monolithic semiconductor structures to two micrometers is a self-aligned titanium silicide process.
In this process, titanium silicide is formed on the upper surface of a polycrystalline silicon (poly) line to dramatically increase the conductivity of the poly line and to obviate the need for an extra masking step to form metal contacts. These lines are generally formed over a field oxide layer formed on the upper surface of a monocrystalline silicon substrate. A pair of poly lines may terminate on the exposed surface of the substrate with a small gap formed between the terminal ends of the lines.
Often it is necessary to prevent titanium silicide formation over selected regions of exposed silicon. A passivating oxide layer is formed over these selected regions and utilized to prevent titanium silicide formation.
For example, a resistor, diode, or other active device may be formed in a region of a poly line. If the titanium silicide layer overcoated this region, then the actlve device would be short circuited. Alternatively, the terminal ends of a pair of lines may form the base and emitter contacts of a bipolar transistor. If the titanium silicide layer overcoated the exposed region of the substrate, disposed between these terminal ends, then the transistor would be shorted. Accordingly, the passivating oxide layer is utilized to prevent titanium silicide formation over these active regions.
In the self-aligned titanium silicide process, the entire upper surface of the structure, including the substrate and poly lines, is overcoated with a layer of titanium. A first region of the titanium layer overcoats the passivating oxide layer while a second region overcoats the exposed surfaces of the polycrystalline silicon lines.
Subsequently, the entire structure is sintered in an ambient atmosphere of a selected gas to convert the titanium disposed over the exposed surface of the polysilicon into titanium silicide.
Ideally, the titanium disposed over the passivating oxide layer is not converted into titanium silicide but remains metallic titanium.
Next, the metallic titanium is selectively etched from the structure. Thus, titanium silicide layers with their edges self-aligned to the edges of the passivating oxide layer are formed.
Unfortunately, the ideal titanium to titanium silicide conversion process described above is not realized in practice. During this conversion process, silicon atoms diffuse vertically and laterally into the titanium layer. It is the vertical diffusion which causes the titanium disposed over the exposed surface of the silicon to convert into titanium silicide. However, the lateral diffusion of the silicon atoms also causes titanium silicide to form over the passivating oxide layer. Unless the conversion rate is carefully controlled a titanium silicide layer may be formed over the passivating oxide layer, thereby forming a titanium silicide connection over the passivating oxide layer and shorting out the active device disposed below the passivating layer.
Additionally, silicon oxide itself reacts with the metallic titanium to form various conducting compounds. These compounds are not completely removed from the surface of the oxide during the selective etch process. Thus, leakage currents from either the transistor or the active device diminish the performance of the structure.
Accordingly, new processes are being actively developed to reduce lateral diffusion and prevent the formation of leakage currents.